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The massive earthquake, which the U.S. Geological Survey revised to a magnitude of 9.0, from 8.9, and the 23-foot tsunami that devastated Japan on Friday, March 11, 2011, has also caused a nuclear crisis. Cooling systems at the Fukushima Daiichi Nuclear Power Station failed shortly after the earthquake. On Saturday, March 12, there was an explosion at reactor No. 1. Radioactive material was detected outside the plant. Officials feared that a meltdown may occur. By Sunday, officials believed that partial meltdowns had occurred in reactors No. 1 and No. 3. The cooling systems at Fukushima Daini, another plant, had also failed. More than 200,000 residents were evacuated from areas surrounding both facilities.

A Second Explosion

Trouble continued on Monday, March 14, as a second explosion occurred, this time in reactor No. 3 of the Fukushima Daiichi plant, damaging the structure housing the reactor. Water levels dropped at reactor No. 2, exposing the fuel rods, causing it to overheat and raising the threat of a meltdown. Officials continued to pour sea water into the three reactors at the plant. After Monday's explosion, hundreds of people were ordered to stay indoors.

Workers Risk Their Lives to Regain Control

On March 15, the nuclear crisis escalated as officials and workers struggled to regain control over the damaged nuclear reactors after a new explosion and fire. The third blast in four days at the plant happened inside reactor No. 2 while reactor No. 4, which was not operational at the time of the earthquake, caught fire. Engineers at the plant, working at enormous personal risk, continued efforts to cool down No. 2, the most damaged reactor. Most of the 800 workers at the plant had been removed, leaving around 50 to use firefighting equipment in the continuing effort to keep the cores of three reactors cooled by pumping in seawater.

Radioactivity Released Into Atmosphere

Crews put out the fire at the No. 4 reactor just after noon on March 15. The fire was caused by rising temperatures at the reactor's fuel pool, which is not covered. The incident released radioactivity directly into the atmosphere. The Japanese government told people living within 20 miles of the Daiichi plant to stay indoors, stop using air conditioning, and to keep their windows closed.

Radiation Levels Rise Forcing Workers to Retreat

Reactor No. 4 erupted in flames again on March 16th. Officials also announced that reactor No. 3 may have ruptured and appeared to be releasing radioactive steam. Rising radiation levels forced safety workers to temporarily withdraw from the plant for 45 minutes, but water pumps were left running at reactors 1, 2, and 3 to keep them cool. According to Tokyo Electric Power, the plant's operator, five workers died and 22 more have suffered various injuries since the earthquake.

On April 12, Japan raised its assessment of the situation at the Fukushima Daiichi nuclear power plant to Level 7, the worst rating on the international scale, putting the disaster on par with the 1986 Chernobyl explosion. Developed by the International Atomic Energy Agency (IAEA) along with countries who use nuclear energy, the scale defines level 7 as a nuclear accident that involves "widespread health and environmental effects" and the "external release of a significant fraction of the reactor core inventory." Almost two months later, the IAEA called the status of the Fukushima Daiichi nuclear plant "very serious."

What Is a Nuclear Meltdown?

A nuclear meltdown happens when the core of a nuclear reactor is damaged from overheating. It is an informal term, not officially defined by the International Atomic Energy Agency. Meltdowns occur when a nuclear power plant system fails to properly cool the reactor's core, causing fuel rods within the reactor to overheat and melt. Meltdowns are serious because radioactive materials could be released into the environment. After the meltdown, the reactor remains unstable until it is repaired.

What Is Acute Radiation Syndrome?

Acute radiation syndrome (ARS) occurs when the body is exposed to very high doses of ionizing radiation. Also known as radiation poisoning or radiation sickness, ARS is a serious, but treatable disease that affects the digestive system, the skin, and hair. The severity of symptoms directly correlates to the amount of radiation absorbed by the body. Symptoms to the digestive system can begin within minutes and up to days after exposure and include nausea, vomiting, and diarrhea. Swelling, itching, hair loss, and a burn similar to severe sunburn can appear within a few hours after exposure and last a few weeks to a few years, depending on the level of exposure. In order to develop ARS the radiation must penetrate the body and reach the internal organs. Radiation from X-rays and CT scans are typically too low to cause ARS. Treatment consists of controlling pain, treating organs that have been damaged, and preventing further contamination. As a precautionary measure, Japan distributed 230,000 units of potassium iodine to evacuation centers. Potassium iodine can be used for protection against thyroid cancer in the case of radiation exposure, but it does not protect the rest of the body.

Is This Another Chernobyl?

The ongoing situation at the Fukushima Daiichi plant was the most serious nuclear accident since Chernobyl, but lessons learned from that April 1986 disaster helped contain the damage at Fukushima. Chernobyl's meltdown occurred because the reactor blew the unit's casing apart, exposing the core to the atmosphere. Evacuation of the Chernobyl area did not begin until a full 24 hours after the incident. Japanese authorities evacuated 200,000 people from the area of Fukushima within hours of the initial alert. Russia sent two teams of emergency rescue specialists to Japan and made extra deliveries of natural gas, but perhaps their biggest contribution was the lesson learned from the disaster at Chernobyl. In June 2011, Japanese officials said that the radioactive emissions from the Fukushima Daiichi plant in the days after the March 11 earthquake and tsunami disaster might have been more than twice as large as previous estimates. The delay and uncertainty in the report added to the frustration and suspicion of some critics who accused officials of withholding information about the incident from the public.

In December 2011, Prime Minister Yoshihiko Noda declared an end to the worst nuclear disaster since Chernobyl, announcing that workers had all the reactors at the Fukushima Daiichi Nuclear Power Plant under control. However, some radiation areas could take decades to cleanup. Japan's government has budgeted 1.15 trillion yen ($14 billion) through March 2014 for the radiation cleanup, but it is highly likely that the process will take longer, costing more. Throughout 2011, the nuclear disaster sparked anti-nuclear power plant protests and rallies in the country. Early in 2012, former Prime Minister Naoto Kan said, "Japan needs to dramatically reduce its dependence on nuclear power, which supplied 30 percent of its electricity before the crisis." In late February 2012, a new report by the Rebuild Japan Initiative Foundation, found that during the nuclear disaster, no one knew the extent of the damage at the plant. The report also found that while leaders downplayed the risks to the public, they secretly considered evacuating Tokyo.